When did folks realize the true power of "code is data"--that a homoiconic language would let you build a powerful macro system? I don't really get the impression that that idea had taken hold at the time of this paper.
I'm sure that there are many, many folks that haven't realized the true power of "code is data" yet as of now. It is just not that easy to fit in your head especially if you come from a static language.
Back in the day on punch cards etc code was often data. To fit everything on a punch card often times you would use tricks like dividing by 13 rather than 12 (it was close enough) because elsewhere you needed a 13. So a language that combines the two don't seem so out of left field as it does today.
In "The Evolution of Lisp" they mention this AI Memo by Timothy Hart which is from 1963: ftp://publications.ai.mit.edu/ai-publications/pdf/AIM-057.pdf